Generally, as an external recording medium for a data backup of a computer and the like, there exists a magnetic tape cartridge in which a single reel winding a magnetic tape is housed (for example, see FIG. 2 in Japan paten laid open 11-265560).
There exist various kinds of the magnetic tape cartridges, and in recent years, one which forms an opening at a corner of a cartridge case to make a housing space of each device small in a magnetic tape drive and opens/closes the opening by a curved door has been developed. In these magnetic tape cartridges to open/close the curved door, an opening/closing structure is thought of by making the door a free-slidable structure for a case, protrusions such as pins being provided with the door and case, always energizing the door in a closing direction by hooking a tensile coiled spring inbetween these, and operating the protrusions formed on an outer surface from outside as needed. As a method to automate door assembling work in such the structure, after firstly setting the door in a guide groove formed in the case, one method is thought of to assemble a coiled spring of which both ends are pinched/held at the protrusions of the door and case by a robot hand.
However, in the above mentioned structure, if a firstly set door teeters, positions of the protrusions are displaced and the assembling work of the coiled spring becomes difficult, thereby the automation being worried to become difficult. Moreover, if a clearance between the protrusions of the door and case are small, it becomes difficult to insert the robot hand to attach an end of the coiled spring between them, thereby the automation thereof being also worried to become difficult. For such the problems, although an assembling method is thought of, with one end of the coiled spring being trial-fitted on the protrusion of the case, to pinch/hold it by the robot hand and to assemble the other end of the coiled spring in the protrusion of the case, the method is thought to have a tendency resulting in a assembling mistake because it is difficult to keep the coiled spring at a constant position due to its shape.